Colorful Laser Aims at Inspection, Displays

Michael D. Wheeler

LOS ALAMOS, N.M. -- Laser displays and semiconductor inspection could be the future applications for a fiber-based solid-state laser that emits in the red, orange, green or blue.
The discovery was, in large part, an accident. Timothy Gosnell and Ping Xie of the Los Alamos National Laboratory were conducting routine experiments on light amplification. They reproduced a several-year-old French experiment in which a near-infrared laser beam pumped a ZBLAN optical fiber doped with triply-charged ytterbium (Yb³⁺) and praseodymium (Pr³⁺) ions. The French experiment lased relatively weakly at red wavelengths.

Triggered an avalanche
Gosnell and Xie hypothesized that the ions were interacting with each other and with the pump laser, creating a photon avalanche. When they directed a near-infrared beam through the glass fiber, a Yb³⁺ ion transferred its energy to a ground-state Pr³⁺ ion, producing two high-energy Pr³⁺ ions. They sacrificed some energy to produce four high-energy Pr³⁺ ions that, in turn, produced ions and so on. The highly-excited Pr³⁺ eventually released energy as light.
The researchers found that increasing the percentages of both ions would produce more light. Also, by varying the fiber lengths, they could tune the device; a 60-cm fiber produced red and orange light, a 42-cm fiber green light, and a 26-cm fiber blue.
Such a laser could be useful in frequency doubling and semiconductor wafer inspection, according to Gosnell. But at this stage, ZBLAN fiber sells for $2000/m. At that cost, few biomedical or industrial applications seem feasible.
Another problem is that of aligning the pump beam to the fiber core. And though the laser produces blue light, its wavelength is too long for most applications in the blue range.